1. Field of the Invention
The present invention relates to an image forming apparatus adapted for use for example in a copier, and more particularly to an image forming apparatus utilizing the slit scanning exposure method.
2. Description of the Prior Art
For the scanning of an original in a copier utilizing the slit exposure method, there is usually employed, as a scanning exposure means, a movable optical system in which an illuminating system and mirrors are reciprocated with respect to a stationary original placed on a fixed original carriage, or a movable original carriage which is reciprocated holding an original thereon while said mirrors are maintained stationary.
Such scanning exposure means is driven by a motor exclusively provided for this purpose, or by an electromagnetic clutch which selectively transmits the rotation of a main motor maintained in continuous rotation. In either case, and particularly in the case of drive with an electromagnetic clutch, there results a drawback in that the scanning means generates a vibration in the advancing direction thereof by the rapid clutching operation as said scanning means inevitably contains certain vibrational system. This tendency becomes more acute when the scanning speed of said scanning means is larger. This difficulty has conventionally been prevented by starting the scanning exposure of the original after said vibration is sufficiently attenuated, namely by extending the distance of the pre-exposure run of the scanning means, or, if this is not possible, by selecting the scanning speed of said scanning means below a certain limit.